1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a bubble-jet type ink-jet printhead having a hemispherical ink chamber and a manufacturing method thereof.
2. Description of the Related Art
Ink-jet printing heads are devices for printing a predetermined color image by ejecting small droplets of printing ink at desired positions on a recording sheet. Ink ejection mechanisms of an ink-jet printer are generally categorized into two types: an electro-thermal transducer type (bubble-jet type), in which a heat source is employed to form a bubble in ink causing an ink droplet to be ejected, and an electromechanical transducer type, in which a piezoelectric crystal bends to change the volume of ink causing an ink droplet to be expelled.
FIG. 1A is a cross-sectional, perspective view showing an example of the structure of a conventional bubble-jet type ink-jet printhead as disclosed in U.S. Pat. No. 4,882,595. FIG. 1B is a cross-sectional view illustrating a process of ejecting an ink droplet from the printhead of FIG. 1A. The conventional bubble-jet type ink-jet printhead shown in FIGS. 1A and 1B includes a substrate 10, a barrier wall 12 disposed on the substrate 10 for forming an ink chamber 13 filled with ink 19, a heater 14 disposed in the ink chamber 13, and a nozzle plate 11 having a nozzle 16 for ejecting an ink droplet 19′. The ink 19 is introduced into the ink chamber 13 through an ink feed channel 15, and the ink 19 fills the nozzle 16 connected to the ink chamber 13 by capillary action. In a printhead of the current configuration, if current is supplied to the heater 14, the heater 14 generates heat to form a bubble 18 in the ink 19 within the ink chamber 13. The bubble 18 expands to exert pressure on the ink 19 present in the ink chamber 13, which causes an ink droplet 19′ to be expelled through the nozzle 16. Then, ink 19 is introduced through the ink feed channel 15 to refill the ink chamber 13.
There are multiple factors and parameters to consider in making an ink-jet printhead having a bubble-jet type ink ejector. First, it should be simple to manufacture, have a low manufacturing cost, and be capable of being mass-produced. Second, in order to produce high quality color images, the formation of minute, undesirable satellite ink droplets that usually trail an ejected main ink droplet must be avoided. Third, when ink is ejected from one nozzle or when ink refills an ink chamber after ink ejection, cross-talk with adjacent nozzles, from which no ink is ejected, must also be avoided. To this end, a backflow of ink in a direction opposite to the direction ink is ejected from a nozzle must be prevented during ink ejection. Fourth, for high speed printing, a cycle beginning with ink ejection and ending with ink refill in the ink channel must be carried out in as short a period of time as possible. That is, an operating frequency must be high. Fifth, the printhead needs to have a small thermal load imposed due to heat generated by a heater and the printhead should operate stably for long periods of time at high operating frequencies.
The above requirements, however, tend to conflict with one another. Furthermore, the performance of an ink-jet printhead is closely associated with and affected by the structure and design of an ink chamber, an ink channel, and a heater, as well as by the type of formation and expansion of bubbles, and the relative size of each component.
In an effort to overcome problems related to the above requirements, ink-jet printheads having a variety of structures have been proposed in U.S. Pat. Nos. 4,339,762; 5,760,804; 4,847,630; and 5,850,241 in addition to the above-referenced U.S. Pat. No. 4,882,595; European Patent No. 317,171; and Fan-Gang Tseng, Chang-Jin Kim, and Chih-Ming Ho, “A Novel Microinjector with Virtual Chamber Neck,” IEEE MEMS '98, pp. 57-62. However, ink-jet printheads proposed in the above-mentioned patents and publication may satisfy some of the aforementioned requirements but do not completely provide an improved ink-jet printing approach.
FIG. 2 illustrates a back-shooting type ink ejector of another example of a conventional bubble-jet type ink-jet printhead, as disclosed in IEEE MEMS '98, pp. 57-62. In this configuration, a back-shooting technique refers to an ink ejection mechanism in which an ink droplet is ejected in a direction opposite to the direction in which a bubble expands.
As shown in FIG. 2, in the back-shooting type printhead, a heater 24 is disposed around a nozzle 26 formed in a nozzle plate 21. The heater 24 is connected to an electrode (not shown) for applying current and is protected by a protective layer 27 of a predetermined material formed on the nozzle plate 21. The nozzle plate 21 is formed on a substrate 20 and an ink chamber 23 is formed for each nozzle 26 in the substrate 20. The ink chamber 23 is in flow communication with an ink channel 25 and is filled with ink 29. The protective layer 27 for protecting the heater 24 is coated with an anti-wetting layer 30, thereby repelling the ink 29. In the ink ejector configured as described above, if current is applied across the heater 24, the heater 24 generates heat to form a bubble 28 within the ink 29, thereby filling the ink chamber 23. Then, the bubble 28 continues to expand by the heat supplied from the heater 24 and exerts pressure on the ink 29 within the ink chamber 23, thus causing the ink 29 near the nozzle 26 to be ejected through the nozzle 26 in the form of an ink droplet 29′. Then, ink 29 is absorbed through the ink channel 25 to refill the ink chamber 23.
However, the conventional back-shooting type ink-jet printhead has a problem in that a significant percentage of heat generated by the heater 24 is conducted and absorbed into portions other than the ink 29, such as the anti-wetting layer 30 and the protective layer 27 near the nozzle 26. It is desirable that the heat generated by the heater be used for boiling the ink 29 and forming the bubbles 28. However, a significant amount of heat is absorbed into other portions and the remainder of heat is actually used for forming the bubbles 28, thereby wasting energy supplied to form the bubble 28 and consequently degrading energy efficiency. This also increases the period from formation to collapse of the bubble 28. Thus, it is difficult to operate the ink-jet printerhead at a high frequency.
Furthermore, the heat conducted to other portions significantly increases the temperature of the overall printhead as a print cycle runs thereby making long-time stable operation of the printhead difficult due to significant thermal problems. For example, the heat produced by the heater is easily conducted to the surface near the nozzle 26 to increase the temperature of that portion excessively, thereby burning the anti-wetting layer 30 near the nozzle 26 and changing the physical properties of the anti-wetting layer 30.